This invention relates to optical fiber gratings and, in particular, to tunable optical fiber gratings devices.
UV light can induce a permanent refractive index change in some kind optical fibers and optical wave-guides. The photosensitivity of the certain kind optical fiber wave-guide can be used to make Bragg gratings and long period gratings, which is a permanent, spatially periodic refractive index modulation along the length of the photosensitive core of the optical fiber or optic wave guide. Fiber Bragg gratings can selectively reflect specific wavelengths of light within an optical fiber. The selective reflected wavelength is equal to the twice the periods of the Bragg grating times the effective refractive index of the propagation mode. Fiber grating have many applications including band rejection filter, semiconductor laser stabilizer, fiber laser wavelength selector, fiber amplifier reflector, fiber dispersion compensation, DWDM (Drop Wavelength Division Multiplex) filter, WDM add and drop multiplex, light pulse shape reforming, optical fiber switch, optical sensor.
There is a demand to alter the periodic spacing of fiber refractive index perturbations in fiber core (or both core and cladding) to have tunable fiber grating whose wavelength can be controllable. The applications like tunable DWDM filter, dynamic DWDM add and drop multiplexer, tunable fiber laser, tunable wavelength selective switch require tunable fiber Bragg gratings. One attempt is to use magnetostrictive strain for tuning the fiber grating (U.S. Pat. No. 5,812,711). The disadvantages of this approach are that the size of large magnetostrictive component is not small and the cost of the device is relatively high.
G.A. Ball and W.W. Morey used compression-tuned approach to tune fiber Bragg gratings over 32 nm ranges (see G. A. Ball and W. W. Morey Optics Letters, Vol. 19, pp. 1979(1994)). This approach needs very precisely grounded ceramic ferrules, and very high accurate alignment and is very expensive.
Benjamin L. Eggleton, John A. Rogers, Paul S. Westbrook and Thomas A. Strasser from Bell Lab, Lucent Technologies Inc. coated fibers with two metal layers, one uniform metal layer and one variable thickness metal layer along the fiber grating, to tune the fiber grating center wavelength and chirp independently. It is very complicated and costly to coat fiber with two different metal layers.
An object of the present invention is to provide a tunable optical fiber gratings device that obviates the above-mentioned disadvantages.
Another object of the present invention is to provide a tunable optical fiber gratings device with a simple tuning mechanism.
A further object of the present invention is to provide a tunable optical fiber gratings device, which allowed having a wide tuning range with high accuracy.
Still another object of the present invention is to provide a tunable optical fiber gratings device having a low cost.
Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying drawings.
According to the present invention, there is provided a tunable optical fiber gratings device for tuning optical characteristic responses of a longitudinal optical grating area of an optical fiber, the device comprises an elongated beam member defining a neutral plane with a first and a second ends and adapted to receive the fiber therealong, a securing member for continuously securing the optical grating area all along the beam member between the first and second ends and generally parallel to the neutral plane to allow for transmission of a bend of the beam member about the neutral plane to the optical grating area, a fixed support member having a clamping means for releasably securing the first end of the beam member within the neutral plane, and a mobile support member having a directing means for slidably receiving the second end of the beam member within the neutral plane. The mobile support member displaced the second end relative to the first end substantially perpendicularly to the neutral plane and bending the same, thereby stretching or compressing the grating area of the fiber for tuning the optical characteristic responses of the optical fiber depending on a direction of the bend.
Preferably, the optical fiber is a combination of at least one doped fiber zone and at least one fiber grating zone which being adjacent to each other.
Preferably, the beam member has a uniform cross-section perpendicular to the neutral plane between the first and second ends. The beam member preferably has a generally polygonal shape within the neutral plane for providing a non chirped tuning of the fiber.
Preferably, the beam member has at least one guiding member extending between the first and second ends and adapted for guiding the optical grating area of the fiber therealong.
Preferably, the first end of the beam member has at least one abutment member adjacent to the securing member and the fiber is in abutting position with the clamping means for preventing the optical grating area and the securing member from being strained by the clamping means.
Preferably, the second end of the beam member has at least one abutment member adjacent to the securing member and the fiber is in abutting position with the directing means for preventing the optical grating area and the securing member from being strained by the directing means.
Alternatively, the clamping means has at least one abutment member being in abutting position with the first end of the beam member adjacent to the securing member for preventing the optical grating area and the securing member from being strained by the clamping means. Also, the directing means has at least one abutment member being in abutting position with the second end of the beam member adjacent to the securing member for preventing the optical grating area and the securing member from being strained by the directing means.
Preferably, the device further comprises a driving mechanism connected to the mobile support member for displacing the second end relative to the first end of the beam member substantially perpendicularly to the neutral plane via the directing means mounted on the mobile support member.
Preferably, the mobile support member includes a sliding gauge for measuring displacement of the second end of the beam member relative to the first end.
Preferably, the device further comprises a controller connected to the driving mechanism for controlling the bend of the beam member, thereby controlling the optical characteristic responses of the grating area of the optical fiber.
Preferably, the fiber is a first fiber located at a first determined distance from the neutral plane. Preferably, the beam member adapted to receive a second longitudinal optical grating area of a second optical fiber therealong. Preferably, the device further comprises a second securing member for continuously securing the second grating area all along the beam member between the first and second ends and generally parallel to the neutral plane at a second distance from the same.
Preferably, the second fiber secured on an opposite side of the neutral plane relative to the first fiber.
Preferably the first and second distances are approximately equal.